Material is traditionally dried by a drying medium in the form of a gas, the supply temperature of which it is attempted to be kept high in order to minimize the costs for the drying apparatus itself, i.e. chiefly the size of the apparatus. In such a case the drying medium will depart from the apparatus at a relatively high temperature, and above all in a moisture-unsaturated state, which means that it is seldom possible to achieve the optimum energy utilisation.
In order to achieve the greatest possible contact area between material and drying medium the material is often dried in a suspension of said material and said drying medium. This often results in large investments for separating the drying medium and the dried material. Favourable moisture content and temperature gradients between drying medium and material can seldom be reached in suspension drying, due to an uncontrolled mixing of the material. A drying technique of the type just mentioned is described e.g. in the Swedish patent application Nos. 7810558-2 and 8307170-4.
Another known method of drying bark comprises spreading out a layer of moist bark to form a bed on a gas permeable support and passing a drying medium upwards through said support and said bed, as well as linearly moving the bed over the support through which drying medium is blown, new bark being supplied to the bed at its rear end and dried material being removed at its front end. Problems will then occur, since the bed at its front "dried" end has a certain moisture gradient or profile, which means that the moisture content of the removed material is some kind of a mean profile value. Additionally, since the flow resistance of the bed often declines with dropping moisture content therein, the drying medium rather will tend to flow through the relatively dry front or forward part of the bed, the result being that the drying process "gallops" at the front end part of the bed. In turn, this results in a number of drawbacks, e.g. a low moisture content in a large proportion of the drying medium departing from the bed, bringing the requirement for large apparatuses and large energy consumptions for heating and/or dehumidifying the drying medium, if it is to be recirculated through the bed. Furthermore, it is difficult to regulate drying such that removed bed material has the optimum moistness. The temperature and moisture gradients between drying medium and material will also not be the optimum one.
As other prior art which shows other feeding and removal techniques per se reference can be made to FI 66485 which describes an apparatus that works with an endless wire to discharge the material to be dried. From the figure it can be gathered that a thin layer is removed from the bottom of the bed by the means 13 and 14 which are operated in a noncontinuous manner. To operate such a process continuously it should most probably be necessary to remove layers that are very thin which might be possible for very homogeneous materials like cereals or similar but not for bark or similar materials.
Accordingly, one object of the invention is to provide a technique by which the material, arranged as a bed, is dried in conditions such that the drying medium departing from or leaving the bed has a substantially constant or uniform moisture content and is preferably saturated with moisture, while at the same time material with a desired degree of dryness can be taken from the bed.